Many patients at risk of cardiac ischemia have pacemakers, ICDs or other medical devices implanted therein. Cardiac ischemia is a severe condition and great efforts has therefore been made within the medical community to find systems and methods for detecting and monitoring ischemia over time. Electrocardiograms (ECG) are useful for diagnosing ischemia and locate damaged areas within the heart. Cardiac ischemia is a condition whereby heart tissue does not receive adequate amounts of oxygen and is usually caused by a blockage of an artery leading to damage of heart tissue. ECGs are composed of various waves and segments that represent the heart depolarization and repolarization. The ST segment represent the portion of the cardiac signal between ventricular depolarization and ventricular repolarization.
In the prior art, there exist techniques for detecting cardiac ischemia using implanted medical devices. In some conventional IEGM-based ischemia detection techniques, changes in the elevation or depression of the ST segment from a IEGM baseline are used an indication of ischemia. Elevation or depression of the ST segment in an IEGM signal may be the result of abnormalities in the polarization of cardiac tissue during an acute myocardial infraction (MI). An ST segment shift arises because the differences in the electric potential between cells that have become ischemic and those cells are still receiving normal blood flow. Deviation of an ST segment from a baseline is a result of an injury to the cardiac muscle, changes in the synchronization of ventricular muscle depolarization, drug or electrolyte influences, or the like.
In some prior art methods for determining ST window for ischemia detection, the ST window is a fixed time interval relative the R-wave. This may lead to that the ST window may encompass parts of the T-wave or the R-wave. If changes occur to the amplitude of the T-wave or R-wave, this may result in a false indication of an ST episode. Accordingly, there is a need of improving the specificity of these prior art methods. Such improvement can be achieved by allowing manual adjustments of the default parameters defining e.g. the fixed time interval to adapt the parameters to a specific patient.
In U.S. Pat. No. 7,865,232 to Krishnaswamy et al., a method and system for automatically determining ischemia detection parameters is disclosed. An ischemia detection window is based on physiological state indicators that define start and end of the ischemia detection window. The physiological state indicators can be located by identifying slope changes after the R-wave and before the T-wave, respectively. Slope changes are recognized by identifying when the derivative of the composite intrinsic baseline changes sign from positive to negative or vice versa following the R-wave marker. The slope changes are used to locate ischemia detection parameters (e.g. start and end of ST window). A first ischemia detection parameter (indicating the start of the ST window) can be identified as a point along the baseline following the first slope change but with a predetermined offset (e.g. about 25 msec). A second ischemia detection parameter (indicating the end of the ST window) can be identified as a point along the baseline preceding the third slope change with a negative offset (e.g. about 35 msec).
However, there is still a need within the art for a patient-specific determination of ST windows or segments for use in ischemia detection in order to inter alia improve the specificity of the ischemia detection.